質問者様のお求めの英語は「It's been 3 months since I started DMM Eikaiwa」です。
何かを初めてから〇〇と言う場合は、「It's been XX months since」を使えます。
他の事例も挙げてみました：
- It's been 3 months since I last had a cigarette
- It's been 3 months since I broke up with my girlfriend
「DMM英会話を始めて3カ月です。」は”It's been 3 months since I started DMMeikaiwa. ”で大丈夫です。
「～を始めて～～です。」は”It's been ~since I started ~"になります。
「DMM英会話」は名前ですのでそのまま”DMMeikaiwa"になります。
普段、「英会話」を英語で”English conversation"と言います。
ご参考になれば幸いです。
1. I've been taking DMM conversation lessons for the past three months.
2. I started DMM conversation lessons three months ago.
3. I've had lessons on DMM since February.
1. Here, I have used the present perfect continuous to describe an action that began three months before and is still in progress. This is perhaps the most natural way I would express this idea to someone if I wanted them to understand that I'm still taking the lessons.
2. In this sentence I use the simple past because I want to stress the time I began the classes. It is perhaps assumed that I continued with them (maybe I didn't!) but the main point here is about when I started them.
3. In this sentence, by using present perfect, I'm stressing that I began the classes three months ago and have continued up to now. I don't really state that I'm continuing with the classes (I might be though) as here, the message is simply about how long I'v studied for
1. ここでは、現在完了進行形を使って、3か月前に始まり現在も続いている行動を表しています。今もレッスンを受けていることを伝えたいなら、これが最も自然な言い方かもしれません。
2. この文ではレッスンを始めた時期を強調するため単純過去を使っています。おそらく「レッスンを続けているのだろう」と理解されます（続けなかったかもしれませんが！）。ただ、要点はいつ始めたかです。
3. この文では現在完了形を使って、3か月前にレッスンを始めて現在まで続けてきたことを強調しています。ここでは、勉強した期間を伝えようとしているわけなので、"I'm continuing with the classes" とはたぶん言いません（言うかもしれませんが）。
These sentences are stating that one has recently joined/signed up to 'DMM Eikaiwa', To 'start' something is the term used when you have began a process of an activity etc. you are at the very beginning of it unless it is followed with a time period, ex. "I started running 2 weeks ago" "I started cleaning my house".
どちらの文も「最近DMM英会話に加入した」と伝えています。
'start' は（期間を表す語句が続く場合は別ですが）、「～を始めたばかり」という意味を表します。
例：
"I started running 2 weeks ago"（2週間前にランニングを始めました）
"I started cleaning my house"（家の掃除を始めました）
I've been taking lessons with DMM Eikaiwa for three months.
You want to use the present perfect (I have) because this is an action that started in the past and is continuing to happen in the present. You can take lessons for many different things, including English. I hope that this helps. :)
I've been learning English using DMM Eikaiwa for the past three months.
I've been using DMM Eikaiwa for three months now.
Three months ago, I started English lessons with DMM Eikaiwa.
We use the grammatical structure of "have been learning" as a way to explain an action that began in the past and continues to the present day. In this case, what began in the past was learning, and it is still happening now. Any action like this is explained with have - been - verb + ing. If a new student wishes to say this in a simpler way, they can simply say "have been using" and it is implied that through use of an English school, the student is learning English. Also, it is acceptable to start by explaining the timeline and then say "I started" as a way to indicate when classes began.
"have been learning" の文法パターンは、過去に始まったことが現在まで続いていることを表します。この場合は、過去に「勉強」が始まって、それが現在まで続いているということ。このような行動は "have - been - verb + ing" の形で表せます。
よりシンプルに言いたいなら、"have been using" が使えます。英会話スクールを「使って」、英語を学んでいるというニュアンスになります。
また、まず時間について言って、それから "I started" で、それが授業が始まった時であることを表すこともできます。（例3）
It's been three months since I started DMM English lessons.
I've been with DMM for three months.
This let's someone know how many months you have been taking English lessons. You can also be very simple in your speech by saying, I just recently started lessons. You can give a frame of time (3 months) if the person asks "how long". Normally, speaking as a native, we would just say, I've been with DMM for three month. Or, you can rephrase it to say, "Three months ago, I started taking English lessons."
これは、話し手が英語のレッスンを何か月受けているのかを伝えます。
また、シンプルに、
"I just recently started lessons"（最近レッスンを始めました）
のように言うこともできます。
相手が "how long"（どのくらいの間）と聞いてきたら、期間（3か月）を伝えます。
たいていはネイティブスピーカーは、単に、
"I've been with DMM for three month."（DMM英会話のレッスンを3か月受けています）
あるいは、
"Three months ago, I started taking English lessons."（3か月前に、英語のレッスンを始めました）
と言います。
It has been three months since I started taking classes at DMM Eikaiwa
I've been taking classes at DMM Eikaiwa for three months now
When you want to explain that you have been with DMM Eikaiwa for three months; then you have may explain this in the following ways:
-It has been three months since I started taking classes at DMM Eikaiwa
-I've been taking classes at DMM Eikaiwa for three months now
「DMM英会話を始めて3カ月です」は、次のように言えます。
-It has been three months since I started taking classes at DMM Eikaiwa（DMM英会話でレッスンを始めて3カ月になります）
-I've been taking classes at DMM Eikaiwa for three months now（DMM英会話でレッスンを始めて3カ月になります）
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